Automatic alternate routing of interoffice calls



. April 2, 1946. H. H. scHRoEPPEL 2,397,837

AUTOMATIC ALTERNATE ROUTING O'y INTEROFFICE CALLS Filed Oct 5 1944 5 Sheets-Sheet l H. H. scHRoEPPl-:L

AUTOMATIC ALTERNATE ROUTING 0F ITEROFFICE CALLS April 194e.

5 Sheecs-Sheefl 2 Filed Oct. 5, 1944 @www ` INVENTOR Schafe/Upd April 2,'1'946.

H. H. scHRoEPPEL 2,397,887-

AUTOMATIC 'ALTERNATE RUTING OF INTERO-FFOE CALLS Filed Oct. 5. 1944' 5 Sheets-Sheet 3 GPN INVEN TOR Sohrae/Upe ATTORNEY 'April 2, 1946.

H. H. SCHROEPPEL AUTOMATIC ALTERNATE ROUTING 0F INTEROFFICE'CALLS 5 sheets-sheet 4 Filed Oct. 5,' 1944 IN VEN TOR EE Schmepp ATTORNEY April 2,- 1946. l-L H, SCHRQEPPEL I 2,397,887

AUTOMATIC ALTERNATE ROUTING OFl INTEROFFIGE CALLS 5 Sheets-Sheet 5 Filed Oct. 5. 1944 INVENT/OR Schnve/U/oel A f1-l ATTORNEY il llll Il lllllll lllllllmlllllllwlwlllullhlm; d uw] www] im fr@ Lw V1 f1 VWF V. V.

, om! f llll llll l/lumhl l/lsw. wmll llll lll lll Patented Apr. 2, 1946 AUTOMATIC ALTERNATE ROUTINGO INTEROFFICE CALLS Harold H. Schroeppel, Glen Rock, N. J .,assignor Y to American Telephone and `Telegraph Company, a corporation of New York w l' Application October 5, 1944, Serial No. 557,302

9 claims. (c1. 179-18) This invention relates to telephone exchange systems and more particularly to systems in which connections between subscribers lines termlnating in different oflices of an exchange area are established by automatic switching mechanism over groups of interoflice trunks.

In exchange areas having a large number of local ofllces it is not always economical to provide direct groups of trunks for linterconnecting each office with every other oice, particularly where the oices are widely separated and the trailic load is relatively light. In such exchange areas it is sometimes more economical to route some traffic between certain oices through a tandem oflice which serves as a common intermediate -distributing point, so that a number of small, inefficient trunk groups are combined into larger, more efficient groups.

It has heretofore been proposed to render more efficient the trunks in a group directly interconnecting any two ofhces by limiting the number of the direct trunks to the maximum required for handling normal traffic between such ofces. Abnormal or overow traffic between suchoices is then automatically rerouted to a tandem office where the traiiic may be directly routed to the desired oiilce. Where trafiic is increasing this l proposal also may enable existing direct trunk nating office. Each sender has registers for registering both the office and numerical designations of a desired line dialed by a calling subscriber. It also has controlling apparatus for controlling the setting of a district and an oiiice selector to extenda connection to the ofce in which the desired line terminates, and apparatus for controlling other mechanism in the selected office for completing the connection.

A plurality of decoders is also provided common to all ofthe sendersan idle one of which becomes associated with'a sender taken for use by a calling subscriber. The decoder is provided with ofce code registers to which the office code is transferred from the registers of the associated sender. `The decoder is also provided with A,as many route relays as there are trunking points to which calls may be trunked from the originating oice. 'The combined setting of the oice code registers of the decoder determines which one ofi these route relays shall be operated for any particular call. The operated route relay is instrumental in establishing in the associated sender a registration of the classy of the call and a decoded cflice Ycode registration. The sender is thus-'directedto control the setting of proper districtl andjoiiice` selector'switches (or in certain cases av district selector alone), to select I agroup of trunks extending to the ofiice-in which the desired line terminates.

- ITT-plans heretofore proposed forthe alternate routing off interofilce traiic from panel type dial of all trunks of a direct trunk group extending to an oiiioe. This is necessary in order to enable the sender to take the necessary steps to route the connection over an alternate route to the desired cnice in case no idle trunk in the normal group is available. This involves certain problems which will be discussed later.

In systems of the type disclosed in Patentv No. 1,862,549 granted June 14, 1932, to R. Raymond and W. J. Scully, the establishment of connections from an originating office to any other oiilce of the exchange area is made possible by district and office selectors. These are set to select an idle trunk of a group extending to the office in which a desired subscribers line terminates, by a plurality of senders, groups of which are available to all subscribers lines ofthe origiable at thetime vthe route for the call is estab-l offices, provision'has been made for switching from'the group of trunks of the normal route to an alternate route before all kof the trunks of the normal route become busy. This is done in order to reservoir trunks for previously initiated calls which may not yet be completed when the route transfer is made. Such an arrangement may result in Vunnecessarily shifting many calls to an alternate route when asa matter of fact trunks extending over the normal route are availlished. i

InY-accordance with the present invention, a

'plan' isaprovided which permits practically complete Ause Yof all original route trunks before recourse is had to an alternate route. `The scheme herein proposed differs from plans of the prior art principally in that it not only determines the busy condition 'of' trunks actually in use, but lt also provides a temporary record of each call which'may befin course of extension to a trunk group. This recordfwill persistlong enough to ,enable the trunk selection to be completed for Atended over a trunk of the normal trunkfgroup or over an alternate route.

As herein disclosed, the Amaximum sizeiof the original route group may be 23v or'24'trunks, de,- pending upon whether the improved method is applied to an existing decoder iinstallation :or whether it is applied to a new installation, The number of trunks above referred to Wi1l`beade- -quate I:for .most cases. 'However, if...'it abecomes v necessary `to providefor ,largerzgroupa iadditional facilities may readily-,be l'added. llnorder to 1determine `the traffic Y'requirements in this respect, the proposed system provides AAfor va ipeg 'count vof callsvoifered 5to thetrunkgroup :and also'la count vof overflow calls-extended over *the altern'ate route.

Fora clearer understanding of the iinvention, reference may vbe had lto the following vdetailed description taken in connection 4with 1.the accomlpanyingrdrawings in .which Figure 1 shows schematically -fa 'calling and l"a called subscribersl line, together with 'ftrunk Vswitching equipment, senders, Adecoders,feto., rby which :connections .may betextended between "the subscribers;

Fig. 42 `'shows fan arrangement 'lfor determining how many calls lin progress 'arebeingextended to the trunks, andit'alsosh'owsfa portionfof ate'st circuit for ascertaining-whattrunks-are busy;

Fig. 3 shows other portions of the test circuit;

Fig. 'la is 'a modification-of the 'arrangement I Ashown in Fig. 1;, and

Fig. 2a'is a modication oflthatlportion fof the testing equipment f whichisshown Ain 2.

those portions of the overall ycircuitarrangement. vwhich are essential Ito ya proper 'understanding of the invention inthe formit may assume when (new ins'tallationsare tobefmaiie. v'Similaif1y,'Figs.

1a, 2a :and v3 'when laid-'end toend yshow thel 'corresponding features of a modified arrangement adapted tobe applied to existing installations;

SWITCHING vEQIJVIPMEM AND TRUNKSOFFIGS. Y 1,2,.AN11 3 Referring to Fig. 1, a line'nder |0`l 'is'shown for establishing connection *with a :calling `line such as |05! which has initiated 'a call to subscriber station |26 at a-distant oce. uA district selector |02 and an' oflice'selector v|0'| `are provided for Vestablishing connections over a group of normal routing trunks such as ||0 to |23, Jinfclusive. An alternate'route |24 4isf-also provided for the purpose of VVestablishing "a' connection 'to the desired subscriber when vall of the 4'trunks of -the normal group are Ylousy. `Incoming Vselectors such as H0', etc., are'assoeiateii'with-the ydistant ends of the Av-arious trunks and "finaly se` lectors lsuch as |25 lare yprovided to'complete'ithe connection'to the Wantedsubscriber. Y

When a stationfsuch asf 00 starts a call andfa A"lineiindersuch'as |0| has established connection v`therewith, fa district *finder `F03 and va 'lsen'derse-"' 'lector |04 operate to connect a sender |05 to the local trunk terminating in the line nder. A decoder connector |05 operates in a known manner to associate one of the decoders, such as 30030I or 302 to the sender |05. The decoder determines the routing control connections for operating the switches to complete theca-ll. All pt thevswitches, vsenders, decoder-connectors etc.,;shownzschemat icallyin Fig. l'1 are of known type,'andfor a complete description thereof al reference may be had 'to the Patent No. 1,862,549 granted to R.. Ray- 'fmonclrfand W. Scully hereinbefore referred to.

THE DECODER oF FIG. 1

.may be. had :to the laforementionedgpatent. 'The alternate route 4equipment :shown Tat |24V mayloe .ofy lthe type disclosed; in detail fin APatent No, 1,840,132 grantedJanuary '5, 11932, vto T. :II -Roberts. vThe incoming selectors||0 |f|-| .,.etc.and `the final selector 1:25 Vare .of la well-known `type :commonly employedin panel switchingsystems. vilstheir-structural detail and'method of operation form no .p-art of the present .invention,=.they yare V'merely ldisclosed herein schematically.

IIn `order k:that rthe background vof the invention :may be .more 'clearlyfunderstooi leti'us. assume lthat the-subscriber at substation` H10-'of an voriginatingfoflice of 'an exchange area `desirestoies- `tablish :a connection Vwith `the subscribersline |26 which terminates .at a :distant oifice of :the same exchange area. The lcalling subscriber, .uponrremoving his receiver from the switchhook, obtains "a connection with Ian idle sender |05 .in .the originating omce through the operation `of zthelline. finder "l 0 ,l district iinder fl 03 .and sender y:selector l in a well-known manner. The-sub- .':scriber then-dials the oiiice code letters andinulirierical :digits of 'therdirectoryfnumber 7of the lwante'dline |2t.k Y j In response to dialing, Ythe .oilce code andznufmer'i'cal :designations are :registered inthe :sen-der |05 vin a' well-known manner. AFollowi'rigthe reg 'istration `of .the kletters. of the ofceucodedesignation,:and'while the linenumberis dialed, the :decoder connector |06 operatesito'a'ssociatean-:idie fdecoder with :the sender. As xhereinillustrated, three decoders .are provided. Assuming :that .the decoder Y300 is free at'thistimethegdecoder connector |06 causes lthis decoder:to'loe associated With'the `sender |05, and also :operates itheide- IcoderstartirelayST.

`Each `decoder is provided withithexusual frefceiving registers to lwhich yan associated'psender ltransfers the Yletterspf the .oiiicefcodef designation. l.It-,also isV provided withza .routefrelay `correspondi'ngto each route over rwhichconnections', maybe extended from the .originatingoce to. otherfoiices. :In addition, it includes transmittingrregzisters which are selectively set by .anyo'perated of the senderto establish the desire connection through.districtiandoiilceiselectors y Cor through aa ldistrict selector onlyi ,.1 over the :route :indicated -cliy'ithefoperatedirouteLifelayfoflthedecoder.. @my

These one route rela-y Ris illustrated and its connections "are not shovvn as vthey will be understoodfrom vided With a relay OK, which is operated when,

after testing for busy trunks and'calls in progress, it is found that a trunk of the regular group will be available. An alternate route relay AR is also provided, and this relay is operated when, upon test, it is found that no trunk of the normal route 4group will be available.

"Each decoder is also provided with `a release relay RL, which, when operated, permits the i routing information set up by the decoder to'be released to control the operation of the appropriate district selector, oice selector or` other switches, to complete the connection to the oflice of the called subscriber. A test connect relay TK is also included in ther decoder and 'serves to in- I dicate that trunk testing equipment has been associated with the decoder Vfor. testing purposes. Inthe arrangement of Fig. 1, a secondaiy test connect relay TK is arranged to be controlled by the relay TK. Relay TK operates to extend routing connections determined by the route relay R over contacts of said relay R. These connections control the operation of the district selector and ofce selector to extend the call Ato the distant oiiice. Y

As will be described later, the operation is such that, as soon as the decoder obtains access to the testing apparatus, connections for eventually controlling t'ne operation of the district selector and oiice selector are determined by the operated route relay such as R. These connections, how- A ever, do not become effective until the OK relay,

in response to the proper trunk group, indicates that a normal trunk connection is available. If it-should happen that no normal trunk route is available, the AR relay is operated in response to the test, and determines the setting up of a new set of district selector and ofce selector 'control'connections for the alternate route, as

will appear later. Under these conditions the previously established control connections by the route relay R become ineffective. y

TESTING EQUIPMENT or F1Gs.'2 AND 3 trol circuit TCC, which initiates the test and records the result thereof; (2) a trunk testing circuit TTC which determines Whether or not the trunks of a chosen group are busy; and (3) a progressing call test circuit PTC which determines the number of unccmpleted calls in process of be ing routed to the chosen group or trunks.

(a) Decoder PreferemieY circuit The testing apparatus which will be described y Ymore in detail later, has associated therewith a decoder preference circuit DPC which determinesy the order in which several decoders, having access .to the same trunk testing equipment, may obtain control thereof. The decoder preference circuit includes .a series of decoder preference relays f DP1, DPn,- etc. Whose functionxwill appearxlater.Y

Associated with the ,decoder preference circuit DPC is a decoder cut-in relay DC which, after the preference circuit has acted, establishes Va connection from the decoder to the testing .equipment. There is also provided a trunk group cutin relay TC which establishes connections from l trunks of the normal group to the testing equipment for testing purposes. The trunk group cutin relay TC also establishes connections whereby temporary electrical records of calls in progress of completion may be temporarily recorded.

After the operation of decoder preference circuit nthe decoder cut-in relay'DC gives'access to anyfone of 25 different groups ci trunks of. as many as 24 trunks each. This is done by one of 'the contacts of relay DC operating a selected one of the 25 trunk group cut-in relays TC. The operating contact of the relay DC and consequently the TC relay to be selected are determined by the particular route relay of the decoder which is operated in accordance with registered office code letters.

(b) The test control circuit of Fig. 2

The test control circuit TCC includes four relays: (l) an off-normal relay ON for initiating the testing operation; (2) a condenser-timed relay PT, which delays determination of the result oi the test until the various relays of the testing equipment have had an opportunity to operate; (39 a relay OKt which responds when the test indicates that a trunk is available; and (4) an alternate route relay ARt which whenit responds indicates that no trunk is available, and that an alternate route must be established.

(c) (The trunk test circuit of Figs. 2 and 3 `(d) Progressz'ng call test circuit of Fig. 2

The progressing call test circuit PTC is arranged to determine thenumber of calls in process of being routed over trunks of the chosen normal group. The'apparat-us of the progressing call test circuit is arranged on a percentage basis. The idea is toprovide devices for recording an assumed maximum number of previously initiated calls which might be at a given moment in some stage of progress toward. establishing connections to trunks of the group under test. As herein disclosed, it is assumed that the maximum number of calls in progress at any time towardr a particular group of 24 trunks will not exceed four. Therefore, four sets of equipment for recording progressing calls are provided. Only two of these are shown actually connected for use, because in many instances the number of simultaneous progressive calls does not exceed two. However, al1 four sets of equipment may be used if desired.

Each set oi progressing call recording equipment includes a vacuum tube of the triggertype,

which is arranged to break down and ionize when a potential in excess cfa predetermined maximum is appliedito its operating electrodes; iLIn 'the r arrangement illustrated ln Fig 2, fomfssuch 'of these ltubes controls :a -cz'illestorng relay .'lsuch as i CSO, :CS1, etc., each fof which :in fturn tcontrols asubordinate call-.storingfrelay suchcasiCSoQgetc.

v'circuit lWill zbefopera'ted ,nozficall fis in :progress .of completion. Inhoweveufcalls:arerinmogress .on one ior :more ofthetrunks. inthe group-:when

the test lcircuit .out au correspondingmumdaer Iof the .storage l.condensers "Cn, .C i, "etc..wvill have fbeen fdischargedfand will :not yet :have -fc'harged :up 4to Aa potential :sufficient toI foperate the corresponding trigger: tube. Therefore, rif one or more calls are inprog'ress, acorresponding number of trigger 'tubes will fail'to :be operiated. Y

LThe several trigger 'tubes 'control .relaysI such -as CSU, CS1, etc., and these .rela-ys 1in turn operate :subsidiary relays :such iasiCSu'., CS1'.,fe,tc; :The

-zvacuum tubes are less 1in 'number `.than the :num-

Uberof trunksin thefgroup. Therefore, no par- -ticular :vacuum :tube is `.assigned :to any vparticular trunk :for `the purpose of .recording fa, "call in .progress iof completion. .The vacuum tubes and their .associated relays'are .in effect* arrangedin a chainffor .successive operation. When afcall in Vprogress .obtainsv control of the ,progressing call test circuit PTC, and fthe test indicates 'that the 4call VshouldV be routed :over the priginal iroute vby vthe operation of .'a relay O-Kt .as 'hereinafter described, 'th all `will be re istered :u on the .40.

e c g p establishes v-a zcircuit VAoverV test frame Lconductorr irst -dle tube vfrom theleft, for example. "The .term idle as herein Aapplied has reference to a 'tube upon which no call .isrregistered .As will appear later, vvhen a .progressing 'call seizes :the

progressing call test Acircuit PTC, .the .tubes upon?l which Vpreviously initiatedbut uncompleted calls `Nhavebeen registered do .nrrtzoperata but zonly `.the idle tubes operate. The progressing call Vwhich has `seized the testY circuit, however, registers .that call on the first idle tube -'from the'left, by reducing to zero the .voltage :applied 'to .its A'operatf ing electrodes. Y

lWhen the progressing'ca-ll :test circuit PIIIC iis seizedin connection'with acallfor Whichxa trunk `is sought, :all idle "vacuum tubes, :if fany, arent;

once operated andthereupon operate Ltl'le'ir ','correspondingCS relays. .Each CSirrela-y, :whenfop- Y elated, is locked up and vdisconnected'irom.:the

- corresponding'vacuum tube-:so -long'asthe progresing call test circuit is .in operation 'to make ithe,A

necessary test. Thev CSv relays fthus iop'erated, in 4turn operate corresponding CS .rlays, -which Vtogether Vwith the TR :relays of the itrunktest circuit'TTC determine whether :therelis aftrunk 'available or whether the :callrmist:becompletect` 'over'an Valternate route. :In .thesformercasa 'the .operation of the :relay .Olsit Lupon :tire"completion` ofthe test applies :.a. voltagefoverfcontaets'zozthe CS' and :CS :relays associated 'with the :rst foperated tube from the left, to reduce .thenorrnalf- .charging potential of the. :peondenserw-assolciated Awith that itube 'to rzero. Consequently; v`thereon- .denserwilL for:'arpredetermineditimefzthereaiter, l registerra :progressing-r call y:busyceonditioneagainst Ysthegroupr.oritrunks:undertest.

`4Thetestingcircuits, fassa '.whole, including/abe progressing .rfcall .test /circuitlfPliCxand atheitrlmk test circuit ihavefthe contacts ofitheirufe- .laysyso :arrangedV that, `.the number :of 'busy :indicated bytrunk test icircuit SETS. .iplus .thefnumberofmrogressing, calls :indicated-'bythe Yprogr-essing =,cal1 `test Y,circuit fP1TC.is Y.equalatca .or ,greater zhan the totalanumber of trunks Afthe group, the .relay fARtin .theftest controlfcircuit lI-CCf-Will operate. The-calLunder .-testwill there- Y .'ffore.- bemouted over gant-alternate frouterby-thaop- :eration of .the .relay rAR. the decoder.

Y Orres'rrcNlorFresA,i2,ann-i3 Q .Further details .o'f 'the apparatus wllndw .be clearffrom .a detailediiescription of the operation.

VLet us assume vthat thesubscribenatslbsta- .tion ,Ill lconnected to...one .oce of an exchange area ldesires to .establish a, connection with the subscribers line 2`6 whichjterniinates at a distant olice ofthe same exchange area. -Upon. removing his receiver from k.the 's wltclib'ook he .obtains a connection with the 'idle sender I in a wellknown manner 'and 'then dialsthe cnice codeile't- 'ters "and 'the numerical vdigits of the directory' vnumber of'the vdesired line.

In response'to dialing, the oi'lic'e coderandnu- Ymerical designations `are registered inthe sender 105. designation have Abeenso registered the decoder connector 106 'is opera-ted to associate `anridle Adecoder vsuch as "330 `with the-sender. Upon-'the connection of the decoder its fstart relay S'llis operated. Y

(l) Initial operation of decoder .o3..eEzg..` v

ltration tof the rletters of the fo'lce -code 1 designaj tion. Uponclosing itscontact il -4routerelay@R rf, 'from minus 48 `volt battery., 'over contact 2 of :relay AR, contact '-2 :of Irelay 0K, contact ll'of :relay R, conductor Jt'J, zthrough windingA of relay 'DB1 Io'f'the decoder vpreference:circuitiDl-"C,l thence :over rcontact 3 tof :said .relay DPi, and (fassumlng .testing-equipment is .not busy in connection 'withsomefotherdecoderl'zover thexnumb'ericontactstof.zotheradecoder. prefereneefrelayspsuchfas DPmandY DPngi, "to :ground .'If'no @other zde'coder in connection 'with ftheatesting equipment, ithe Vdecoder preference;:relay DB1v associated :With-the decoder 300 is operatedimmediately andzisflacked up over itscontact 3. If the testing4 equipment is associated'with 'another decoder, operation of re- :lay DB1 awaits :releaserof thefclecoder v.preference .relay of :such vother decoder.

Relay .DP1, upon operating, -iconnectsiground over yits lground contact 4 '.to the circuit :tlc which :passes through the trunk y:testy connect'frelay'TK .to battery.v Over its "contact 2,.relay .DB1 -reompletes a circuit. from ground, through the: Winding :of `decoder cut-in .relay DC, .over conductor dc,A i and.` over contact 6 1 olfatl'ie.V off-normalmelayfON. Relay "IK rin the, decoder being. oper-atedit opens at its contact I Athecircuit of therelease relay RL of the decoder Vto 'prevent the decoder 300 .from :transmitting routing, informationV for operating the 4.district fselector; switch :H12 and zoice "selector `switch I E01 iuntiliitihasbeemdetermined whether an idle, trunk; of :the'normalztrunk groupvis available, tor Whetherffan alternate route will have .to 'be-used..

,uit contacta,frelay'fIKzopensfthezconductorrar' itoriprevent.; the; operation tornan alternate :route As soon 'as the letters of 'the 'ofce code relay (not shown) over contact 9 of the normal route relay R. By closing its contact 2, relay TK operates relay TK'y over a circuit from ground, over a contact of the start relay ST, contact 3 of relay AR, contact 2 of relay TK, and through the winding of relay TK to battery. Relay TK is locked over its contact 2 independently of the relay TK, andat its contacts 3 to 8, inclusive, connects ground over the contacts 3 to 8, inclusive, of the route relay R to circuits associated with terminals DB, DG, OB, OG, CL and CR, whereby certain operations relating to district group selection, district brush selection, oice group selection, oiiice brush selection, determination4 of the class of call and selection of a compensating resistance are initiated. These operations are prevented for the moment, however, by the fact that the circuit of the release relay RL is held open at the contact I of the relay TK so that routing information cannot be released for the selection of a trunk. Y

(2) I Trunk group selection in Figs. 1 and 2 'Ihe operation of the decoder cut-in relay DC bythe relay DP1 as above described, sets into operation testing equipment to determine whether an idle trunk is available in the normal trunk group selected, or whether it will be necessary to use analternate route. The operation of the relay DC causes said relay to lock over its contact l independently of the loff-normal relay ON. By closing its contact 2, relay DC connects gro-und to conductor on and through the lower winding of the off-normal relay ON to battery, thus operating relay ON. At its contacts 3 and 4, relay DC prepared circuits over conductors ar and ok, respectively, for a purpose which will be de. scribed later.

In order to determine what groupof trunks will be selected, test decoder cut-in relay DC is provided with a set of 25 additional contacts designated tgv to tgzg inclusive, to select one of 25 trunk group cut-in relays such as TG1. Which one of these contacts will become effective when the, relay DC is operated, depends upon which group relay R was operated in the p-rocess of registering in the decoder routing information determined by the oice code letters. .In the case illustrated, the operated route relay R, at its second contact, completes a circuit over conductor tgl from ground, over contact ll of the alternate routerelay AR (which is not operated at this time),over contact 2 of route relay R, over conductor tgl, `thence over the tgl contact of relay DC, and over conductor tgi through the winding of trunk group cut-in relay TG1 t0 battery.

It will be evident that the operation of the relay DC gives access through 25 trunkgroup cutin relays such as TCr to 25 groups of trunks, each group in the case illustrated including 24 trunks. The 24 trunks selected by the trunk group cut-in relay TG1 have their sleeves connected to the 24 contacts of relay TC1 designated so to szs. These contacts are in turn connected to the windings of thel trunk test relays TRO to TRzs of the trunk test circuit TTC.

(3) Preliminary operation of test control circuit TCC of Figs. 2 and 3 from the various DC relays to prevent the operation of other DC relaysl The actuated relay DC, however, holds up its armature because of its At its contact 3 relay ON connects ground over the number l Contact of 4trunk group cut-in relay TC1 to operate a peg count to record the fact that an additional call has come in to the group of 24 trunks controlled by the relay TG1.

At its contact 4, relay ON connects battery through a resistance and the upper winding of timing relay PT, and thence over contact 3 of relay AR: and Contact 3 of relay OKt to ground, At the same time the biasing current for relay PT will be caused to iiow from battery, over'contact f4 of off-normal relay ON, through the lower or biasing winding of relay PT, and through the condenser Cpt and series resistance to ground. The current through the biasing winding is suiiicient to prevent the operation of the armature l of the relay PT vrby its upper operating winding luntil the condenser has been charged up to a point where very little current ows through the condenser. The operation of the relay PT is therefore delayed long enough to enable the various testing relays to perform their functions as will be described later; The relay PT when ac- .tuated establishes a circuit tt which entends over contacts of the testing relays 4to determine whether the OK signal or an AR signal will be returned to the decoder, as will be described later. The OK signal indicates that one of the selected group of v24: trunks is to be used to complete the call, whereas the sending of the AR signal means that an alternate route must be employed.

(4) Operation of trunk test circuit TTC As already pointed out, the sleeves of the 24 y This sets up a connection Within the trunk test Y circuit TTC, as will be described later, which takes a part in determining whether an OK or an AR signal is to be sent to the decoder. It will thusk be seen that the trunk testr circuit TTC op- 'erates in effect to reco-rd what individual trunks of the selected group of 24 are busy.

In order to make allowances for any calls which have been already initiated and are .in thev course of progress toward the selected trunk group without yet having seized a trunk to make it busy, the invention is arranged to determine how many such calls are in progress. y It is. for this purpose that the progressing call test circuit PTC is provided ashas already beenstated. It consists of a-number of vacuum tubes such as VTn to VTS, inclusive, having associated therewith CS and CS relays. The trunk group cut-in relay TC1 is also provided with contacts such as csa, cs1, cs2, and csa, which lead to suitable resistances Re to Rs, inclusive, and thence through a common resistance to plus volt battery. Bridged to the resistances Rn `to R3, inclusive, are condensers Ct, C1, C2 and n Ca. While four condensers are provided, someor them may be ing voltage will be applied over contact B olrelay OKt, over contact l of relays CSo', CS1', CS2', over contact 2 of relay CSa', over contact 5 of relay CSa, and over contact css of relay TC1 to condenser C3. The call under consideration will, therefore, be registered on condenser Cs.

The foregoing is on the assumption that no preceding call is registered on the condenser Cs. If prior calls still in progress have been registered on all four condensers, none of the four vacuum tubes would operate. Consequently, none of the relays CSo' to CSs would be operated, In this case the call under consideration would be registered byconnecting negative 48 volt battery over contact 6 of relay OKt, over contact |`of relays CSQ to CSa', inclusive, thence over conductor cs, and over contact of relay CSo, to condenser Co. If at this time the condenser C o was partly discharged, but insuiciently charged to operate the vacuum tube VTo as a result of a previously registered call, the condenser Cn will be again fully discharged to register anew lcall in progress.

(6) Final operation of test control circuit TCC of Figs. 2 cmd 3 to choose alternate route 'signal or an AR signal is to be sent to the decoder It has already been pointed out that the operation of the relay PT is delayed by the action of the current flowing through its biasing Winding to charge the condenser Cpt. As soon as the relay PT operates, however, it closes a circuit from ground over its front contact and through the upper winding of the oil-normal relay ON to hold the off-normal relay operated. The relay PT also connects ground over its front contact to the conductor tt and thus extends a circuit over said conductor and the contacts of the TR' relays and CS relays to operate either the relay ARt or the relay OKt.

Let us assume that all of the trunks are busy except trunks number 0 and number 22, and that the progressing call test circuit PTC indicates that four calls are progressing toward the trunks. Under these conditions. the circuit extending from the contact of relay PT over the conductor tt will be extended over contact l of the relay r'Rn'. contact 9 of relay TR1', contact 4 of relay TR2', contact 4 of relay TRs, contact 4 of relay TR4', and thence over contact 4 of other TR relays until the circuit arrives at the contact of the relay TRzz, which is not operated. At this point thecircuit will not pass over contact 4 of relay TR22, but over the back contact 3 of said relay, and thence over front contactl 6 of the operated relay TRza' to conductor c, Since four progressing calls are ground. At its contact 2, relay ARt grounds the ar lead to the decoder 300 from ground at contact 3 of relay OKz, over contact 2 of relay ARt, over conductor ar, and over contact 3 of relay DC.

From this point it continues over conductor ar into the decoder Where the circuit passes through thewinding of relay AR in the decoder to battery. The result of the operation of the l relay AR in the decoder will be described later.

Returning to relay ARt in the test controlling circuit TCC, at its contact 3 said relay opens the operating Winding of relay PT. Also by closing its contact 4, relay ARt connects ground to the condenser Cpt to discharge said condenser in prepwill be recalled at this point that the trunk peg registered in the progressing call test circuit PTC, l

none of the CS' relays are operated. Consequently, the circuit continues from conductor c over contact 5 of relay CSs, contact 3 of relay CS2' to conductor a, and thence over contact 5 of relay OKs and through the winding of relay ARt to battery.

The relay ARt, on being operated, locks over its contact I, and over contact 2V of relay ON, to

count has already been registered by the operation of the off-normal relay ON to close its contact 3.

Returning again to relay PT, which was releasedby the opening of contact 3 of relay ARt, it will be noted that the release of relay PT disconnects ground from the upper Winding of relay ON. Said relay ON still remains operated, however, through its lower winding and over the conductor on and over contact 2 of relay DC to ground.

(7) Final operation of decoder of Fig. 1 to choose alternate route Returning now to relay AR in the decoder, l

which, it will be recalled, was operated by the relay ARt in the test control circuit TCC, it will be noted that the relay AR locks up to ground over its contact I and over the contact of relay ST. By opening its contact 2, relay AR disconnects battery from the circuit extending over contact 2 of relay OK and over contact I of the route relay R to the conductor tf, and thence through the Winding of the relay DPi in the decoder preference circuit. The latter relay is released with results which will be presently described.

At its contact 3, relay AR opens the locking cir cuit of the relay TK', causing the release of said relay and consequently the opening of paths DB, DG, OB, OG, CL and CR. This prevents routing the call over any of the trunks of the selected groups H0, III, etc. Relay AR, by opening its contact 4 disconnects ground from the path over the contact 2 of relay R, over conductor tgl, over a contact of the relay DC, and thence over conductor tgi' and through the winding of the relay TG1 to battery, thus releasing the trunk group cut-in relay TG1.

Relay R, at its contact 5 connects ground to a circuit leading over back contact 3 of relay TK (which has in the meantime been released by the release of the relay DP1) and thence over conductor ar' and contact 9 of route relayR to operate an alternate route relay (not shown) in the decoder 300. The alternate route relay then proceeds to operate in the usual manner, causing the district selector |02 to select the alternate route |24. The call will then be completed over the incoming selector |24', and selector I 25 to the calling subscribers station |26. The relay Alt is then released by the deenergization of the start relay srfimthelreauiar course. 'ofi the completion i offthefcalli Titre .decoder thus; restored'tonormal;m.a.well;known manner.`

Returning; tothefrelay DPL 'which it; will; be,V recalledwas. releasedby the. operationrof the: relay: AR',;said; relay DPi, inr addition toy releasingthe-z relay TK inthe. decoder-ants; contact ;;o-pensfat; Contact 2. This disconnects ground from ilnefcir.-y cuit/off' relayfDC., which isreleasedythereby ref-l storingv the decoder; preference circuit tof normal.

They release of relay DCI,y by openingits con.- tact. 2',Y disconnects ground: fromfthe lower-Windfing of thefof-normalrelay'ON so that thezlatterf' is released. Relay ON, byy opening its Contact4 2 l' disconnectslcckirie ground from-thefrelay ARtA inv thetestfcontrclfcircuitzand from any CS'andsCSf relayswhich have been operatedin the progressing call'testcircuitivPTC. lt will be'reinem-biered,.`

however,- rthat .underxtheeonditions assumed; none.Y of the. Vacuurntubes VT() to; VTav was operated. Therefore, no- CS or'CS' relays` had. beenxoperated; Ii, however,y anyhad been operated, as.. might be the'. case 'if less' than four callsr were in.

progress when: the trunk group cut-fin relay TCL was. energized', suchrelaysfwould have been rem leased.l

The release ofthe relay TG1, already described,

disconnects` all. oijthesleevei conductors of the selected: group ofttrunks from the relays; So to S23, inclusive; Thus all of the relaysinthe, trunk. 30 test'. circuit; TTC: are released and the.` apparatus is restored to'ncrma-l.

(8) Finalopeation of. testcontrolezrcuzt of Fig. 2 when normal route trunk is available The. foregoing isan; example of'. a condition` un'- A typical situation resulting in an OK Let' us. assume that all' trunks excepting those corresponding to. conductors Se, Si and Snare y l busy, and"v that the .progressive call test circuit" PTC shows. progressing calls registered on Vacu.- um tubes VTc. and VT2 but not on Vacuum tubes l VTi an'dVTs.. Under these conditions, all of the trunkv test relays of the TR group Will be, operi der Which an AR signall will befsent to the del coder.

signal Will nowl be described. 4o

ated except relays TRO', TR1" and TRzz'. Also, `V

tact 9` of relayr TR2", frontA Contact l of. relay TRS', front contact. E of" relay TRif, Yand over'. contact .of succeeding TR* relays until relay "IRza" is reached'. At' said Vrelay the circuit continues from contact 6 of the immediately preceding relay,. over. back contact 5 oi relayk IRzzf ,-1. and.

j thence. over frontV contact. l of relayTRza. to. con-- ductor d'. From. conductor d the circuit proceeds `over contact 'Lof'. relay CSaI, contact. H1 of Vrelay CS2", c-ontact.- 9 ci relay CS1.,andA thence'over. conductor 7c, over contact 5, of, relay ARi, and. through the Winding of, relay OKt to battery.`V

Relay OKt, uponbeng operatedlocks up over. its contact'. 'li to ground over contact. 2 of-.relay/ ON. At4` contact 3l. relay QKi` opensl the. upper operating circuit ofrelayePT, and. by closing conftact. .at 4;, relay OKt, discharges the. condensers,F

o, leases the oir-normal relay ON of' the testv conf Cpe. Relay-slil/'I1is'thereby..I restored toznormal-s Y itscontact: 5 .relay @Kt` opens ,the pathfto; operate: f the relay/ ARr, thereby preventing` false-.operation of;thalat-ter;. Y

ByY closingf contact 6.2 relay @Kr connectsA minus-.- 48 Volt battery; over. contact Lof. relay-CSo?,-,over': contact. 2f ofrelay4 CS1! over contact 5f or relay CS1, through. resistance: m',. over contact csi.' or relayy TEL to; discharge.; the: condenser;v Ci. ThisV registers. a: progressing; call on a: condenser 01 so 2 that on thex nextv operation of: the; progressing."K calli test; circuit. PTCif itY occurs;` within. the time required'.y torechargef condenserto the break-1A down; voltage; ot.l the.l vacuum tube. V'Ii,A saidz tube V'l'lr.andlits:associatedirelaysWilloriotxeoperated;y

(9) Finali operation,- oj decoder of Fig.g Z

normal. route. trunk, is auaz'lab'a the conductor olc, and thence over Contact 4f off relay DC', over tlieA 0k conductor leading int'o the decoder 360, and' through the Winding' of relay OKin the-decoder 3D1) tobattery.= Relay OK upon. being; operated, loicks. upwover: itsy contact L to `ground over contact of relay ST. Y By opening contact 2 rel'ayOK removes battery,I from the conductor'tf vthus releasing the relay-DP1 iny the, decoder` preference.. circuit DPC. At" its" contact 3, relayl OK completes a circuit for the release. relay VRL in the' decoder' 390; This `permits the routing information established by the route relayPv" oyertheV circuitspDB; DG, OBOG, CL and" CR- to eiectivelyfoperatethe district selector H321 and. office selector |07. to route. the callI overv one of the idle. trunks Voithe elusive; The subscribers circuit, of course; is.V completedto the'substation IZB over that Aone oflthe incoming, selector switches HIJ'V to, |23",

V inclusive, associated with the chosen trunk andby. a final selector |25.' The relay GK ofthe decoder,

isl released after thecall" releaselof.` the start relay manner. Y y Y The releasepof the relay DP1. lin theA decoder preference circuit DPC Aopens-.at its. contact' 4i the original energizing circuit of relay TK in. the decodenand'. at'y contact ELopensV` the. circuit. o'f` relay DC.'` The release. of? relay DC in tur-nre,-YK

is completed. by the.. ST' in. a. `Well-known'4 trol circuit TCC. and the.. trunk group cut-in relay TC1. As a consequence,V the testingr apparatus will' berestored to normal in. a manners.V already described.v I It. Will, of.' course,I be.r evidentY that. the, numerous. combinations ol. busytrunks andprog-ressing calls. Will'operate either the relay ARc.or.'the relay, OK@ over various.. circuits.` Without attempting to trace.. all the. possible combinations, it-Willgber clear. how theV relays ARtand OKtoperate. under`y various conditions fromthe followingytablezf Number Grounded' 't .n ANumber of. Relay:

ofidl'e from 'r'ro- Number-0f' csfrel f '(A'R''Ur" v 1 stored calls ys. (0K0. trunks l to RTC y operated. Operated a.- '4-,.3, 2, toro-... cf-4 Aa a `Non@ 4 gonfi b .1, .2, 3 or4 Lessthan 4 (ARQ) c `Notmorethan l. 3cr 4. t (O'KD'v c. `2, 3'or:4; Lessthan 3;-. g (.AR): d Not more than 2 d.' S-or e ot'more thanBl Y e' k 0, l, 2, 3 or 4-..-. 0-4

group of ve final groups.

Monmrn ARRANGEMENT or F1os. 1A, 2A, AND 3 A modified arrangement is shown in Figs. 1a, 2a, and 3. For convenience in tracing the circuits the sheets containing these three figures should be arranged end to end. f

The arrangement shown in these three figures is designed primarily for use in existing decoder installations. To this end the circuit is so designed that it may be readily wired into existing installations and that it may use relays and other equipment of the kind employed in such installa-4 tions. In principle, the system is very similar to that already described, the principal differences being certain changes in the circuit arrangement of the decoder, and also some modification in the method by which a group of trunks is selected. For `selection purposes, the 25 groups of trunks available are divided into 5 subgroups of 5 trunks each. Each group of trunks in this case is 23 vinstead of 24, because one contact of the TC relay which selects a nal group of trunks is used to complete the ar circuit instead of connecting a -test relay of the trunk test circuit TTCl to the sleeves of a trunk.

In order to select a trunk group out of a total of 25, group selector SGS is provided which includes a relay DC. Relay DC has 5 contacts used for selecting one of 5 subgroup relays such as SGo to SG4, inclusive. In addition, the relay DC employs 5 contacts to complete circuits from decoders to 5 contacts 'of each of the SG relays, By means of this arrangement the decoder, in selecting a group of trunks, acts through the sublgroup selector SGS to first select from the set of trunk groups available a limited subsidiary group of 5 trunk groups, from which the final group of `23 trunks is selected in accordance with the particular route relay operated in the decoder.

In general, the test control circuit TCC, the progressing call test circuit PTC, and the trunk test circuit TTC, are the same as in the system already described, and the operation is very similar. So also, the trunks and switches by which connections are extended from subscriber to subscriber are the same in Fig. 1a as in Fig.'1.

Therefore, further details of the apparatus mayA be understood from a description of the operation of those parts of the circuit in Figs. 1a and lb which are different from those in Fig. 1 and Fig. 2, respectively.

(l0) Imtial operation of decoder of Fig. 1a

Fig. la, the operation of the relay R results in the energization of a test circuit finder relay TF, a trunk subgroup relay SGr, and a final trunk group relay TGo over conductors af, at and ag, respectively. Actually, there will be five SG re-` lays, SGo to SG4, inclusive, and flve TG relays, TGo to TG4, inclusive, associated With the decoder such as 300. By arranging matters so that different route relays operate different combinations of these relays in pairs, any desired final group of 23 trunks may be selected. The SG relays are for the purpose of determining the subgroup of five trunk groups out of a total of tWenty-ve groups available, and the TG relays function to select the final group of 23 trunks out of a'sub- Ythe trunk testing'circuit Wanted and the `final trunk group to be tested. The operation of relay SG4 connects ground to the conductor sgr'to de-VVV lrelay TGu over the closed Contact `of relay TF,

over` conductor tf, through the Winding of the decoder preference relay DPi, over Contact 3 of said relay, and over the third contact of other decoder preference relays (if they are not operated) to ground. Relay DP1 operates and locks up Vover -its make contact 3. At'its contact 2 it connects f ground to the circuit leading to relay DC, through the VWinding of said relay, and over conductor dc to battery over the back contact 6 jof off-normal relay ON. Relay DC is thereby operated with results which Willbe described later.

Upon the operation of relay DPi, it connects ground overjits contact 4 tothe conductor tk and through the Winding'of the relay TK to bat-f tery. Relay TK, upon operating, at its contact IV connects ground tothe low resistance Winding of the relay TGo. In this connection it should be noted that the upper winding of the relay TG1 is of `high resistance, and that the decoding relays (not shown)s which are normally operated over contact 2 of the route relay R, are in series with this high resistance Winding. The decoding relays are marginal and hence prevented from operating in series with the high resistance Winding. They ydo not loperate until such time as the low resistance windingof the relay TGo is out in by the operation of the relay TK, as just described. Therefore the decoding operation of the decoder 300 is held up until after the operation of the relay TK, when it is permitted to` leasing the decoded routing information for trunk selection until the trunk testing operations have been completed, as will be described later. By closing its contact 3, relay TK completes a circuit for relay TR from battery, through the winding of said relay, over contact 3 of relay TK, over contact 3 of alternate route relay AR, and thence to ground vover the contact of start relay ST. The operation of relay TR is used to make ineffective the switching controlling circuits DB, DG, OB, OG, CL, and CR associated with the contacts ofV relay AR for ashort time after relay AR is operated and relay R is released. This -occurs if the test circuit indicates that the callV should be directed tothe alternate route, as hereinafter described, to permit Vcancellation of -switchinginformation previously transmitted' to the sender by the like designated Ycontacts of relay R before new switching information is transmitted by the contacts of relay AR.

(11) Selection of final trunk group by subgroup selector SGS in Fig. 2a

Returning now to the relay DC, which it will YI contact. Contact 2 connects' ground to the conductor on toV operate the off-normal relayON 'through'its lower winding. IIt will also be noted that the relay DC has ve contacts connected to 4conductors son to sgrl for the purpose of deter- Fmining which one of the subgrouprelays SG!) `t`ofSGis`ft be Eoper"ated, to tnereby determine a subff'ruTp"ofA venal trunk 'groupsfout of the total of twenty-nyc groups. Since the Eoperation 'of the relay 'SG4 in the decoder 'connected ground to the conductor Sg'at as already described, this grounded connection extends over the Iafi-- propriate --`contact of v the relay DC to operate thesubgrup relaywSGi. y

It will also be noted that the relay DC has '5 contacts connected to conductors tgo to tgt, in"- clusive.` These contacts are used to determine WhichY final group of 23 trunks 'of the 'subgroup of ve nal groups lis :to lbe selected. vvAs 4already described, the operation v'ofl the relay TGb grounded the conductor tgt. This 'ground is eil"- tendedover said conductor, over the approprie ate -'contact Iofthe relay jDC, and thence over contact szo of operated relay ySGA Ito the conductor tgzd". The "circuit Continues yover tHe flatter conductor ythrol'igh the :Winding of the final trunk group out in relay TCzo, which operated.

(l2) Operaionof testing equipment off 2a and 3 when normal route trunk is available The iinal subgroup of trunks has vnon' been 'selected'and the test control 'circuit TCC has been started into operation by the 'energi'zation ofthe "ori-normal relay ON. The delayed action relay PT after Va'short interval is operated 'over vconftact -4 vof the off-normal relay ON, as described in connection 'with Fig. '2.

The operation of the relay TCzu conne'ctsjthe sleeves of the final group of 23 trunks to the TR relays of the trunktest circuit TTC 'in the saine manner as described in v'connection with Figs. 2 vand 3. Likewise, the lvacuum tubes and associated relays of theprogressing call testA circuit vPTCv are operated in accordance Twith VArecorded progressing calls registered on 'thejcondensers Co to Cs, as described 'in connection with Fig. 2

Upon the operation of the relay PT, ground is connected to conductor vtt-and vra circuit is completed from said conductor oVercon'tacts :of the -fTR relays and lTS-relaysfto operate either the relayvOKt or the relay ARt in the test control 'circuit TCC, depending uponfiyh'etheratrunk of the finalgroup of '23 is Iayailab'le, or lWhether "the alternate route has yto be used. yThis,V of course, Vdependsupontlie'c'onribination of TR' 'relays'and TS relays employed, fas has already "been 'fully l -pointed'out in kconnection with Figs. y2 and 3.

Assuming that a trunlg'isavailable and that the relay 0K1; 'ist operated',V the relay OKn, Aby closing lits contact', registers a progressing 'call upon'one of the Vacuum tubes 'at thejprogressingcall'test circuit, in the manner Ypreyiouslydescribed. `At 'its contactvZ, said relay DKr-connects ground t'o 'the conductor ok, which passesiover contact-f4 of relay DC' and fthence into "the decoder 300,

of the relay O f Relay OK inthe decoder fSUUjWhen operated as above describedflocks up over its contact l.

where the conductor oc connects'tothewinding.

' At its contact 2,*re1ay OK-connectsfground vto the lower resistance Windingof relay '1Go-independently Vof the connection heretoforeextending over contact -2 of relay TK. fBy opening the contact 3, relay OK disconnects ,battery from the trunk test Yfinding conductor tf, already ltraced over contacts-offrelays TF and This Areleases the V,decoder preference 'relaylDP-i and the relays controlled thereby.V At its contact 4 relayOKfcloses attacca the circuit of the. release relayzRL of tiffefdecoder., so that the/decoding .process controlled over contact 2.01.?- relay 'TK is now permitted to proceed, thus *establiskiingv a connectionuto the subscriber stationn26 over one of thetrunks Vl'll to |22, inclusive.'Y y f j YAt its Contact@ relay DP; releases tlfiefrela TK. By opening its contact 72, relay DP; disconnects-ground from tn ecircuit of therelayDC and releases thelatter. V.lelay DC, in 4falling o'i, 'releases relay ,'SfGgrancl oli-normal relay L 'The remaining relays of the control circuit TCC, .progressingucalltest circuit PTC and trunk test circuit are restored to'normal inthe manner already described in connection with the Yarrangement of Figs, 1, 2, and 3.

l It lwill be understood, ofcourse, that as the decoding .process proceeds, the ystart relay ST and the route relay R. are released. The lformer opens the lockingoircuit of. relay OK, which is Y restored to normal. The lrelea'se of theroute relay R restores the relays TF, MSCM `.and 'IGo to normal, vso that the decoder lis now 'ready for Va new call.

"(14) operation of aecoer and testing equipyIncase thelrelay Alici in Zagha's ybeen-operated instead of the `relay OKt, relay VAE1; would have locked up and performedother functions Aas already described in vconnection with 'Fig. '2. By closing 'its contact, .it connects ground over contact 3 of relay Okt, over saidcontact of relay Afin, overwconductor an'All over -the upper contact of -relay T029, 'thence' oyer conductor an, over contact 3 of therelay DC, into the decoder 300, Where the conductor ar :is connected to the Windingof the 'alternate route relay Y The relay ARis thereby operated and'locks up o ver its contact l and over a contact ofthe relay ST to ground. By closing contacts l and 2, the "relay VAR also lcompletes a circuit for the relay from .ground, over contact 2 of the relay ST, `over contacts I and 2 ofthe relay AR,-and thence Aover contact i of the relay TK to battery f t'hroug'h the Winding of relay TR. By 'opening its contact '3, relay ARopens theV circuitpreviou'sly traced for relayTlft, whichis now released.

The Aoperation of relay TR atits Contact l releases the route relay R so that the routing determined bythat relay 'will be defeated. l'Re- `vlayfR 'upon releasing'opens vthe operating circuits The release rvoi vre-v vof relays TF, 'SGgand '.IGol

lays TF 'and'SGo releases'the DPre'layfofthelde- 'coder preference circuit, causing vthe vtrunk-'test 'fciicuit -to vrestore to normal :as 'previously 1de- *tofnormaL the decoding of. the alternate :route proceeds. The Trelease "ofrelay closes the --routing information has `.been recorded in vthe sender, relay RL operates and causes thedeooder torestore to normal., f f

While this'inventionhas beenv disclosed incertain specific' arrangements'wnich are deerne'd'de- `sirablel it willbeobvioi'isfthat the .general principles hereinset forth maybe embodied 'in many other organizations,Yv Widely :different from those from said first cnice to said second oiice in response to a code, said decoder including means to determine the group of trunks'over which such connections would normally be routed, means to indicate the busy or idle condition of each trunk of the normal group, means associated with said group of trunks to indicate the number of uncompleted calls in .progress Itoward trunks of said group, testing equipment for testing how many trunks of said group are busy with established calls and how many uncompleted calls already initiated are being routed to trunks of said group, and means whereby said decoder will route a call over alternate facilities when said test indicates that established calls over trunks of 'said group and previously initiated calls being routed thereto will utilize all of the trunks of said group.

2. In a switching system, trunks extending between oliices and over which interoce connections may normally be routed, alternate facilities over which connections between offices may be routed, switches for establishing interomce connections from a calling subscriber at one oflce to a second ofi-lee at which a called subscribers line terminates, a decoder for controlling the operation of said switches to complete a connection from said first oiice to said second oilice in response to a code, said decoder including means to determine the group of ,trunks over which such connections would normally be routed,

means to indicate the busy or idle condition of each trunk of the normal group, means associated with said group of trunks -to indicate the number of uncompleted calls in progress toward trunks of said group, testing equipment for testing how many trunks of said group are busy with established calls and how many uncompleted calls already initiated are being routed to trunks of said group, means whereby said decoder will route a call over alternate facilities when said test indicates that established calls over trunks of said group and previously initiated calls being routed thereto will utilize all of the trunks of said group, and means whereby said decoder will route a call over a trunk of said group when the total number of established calls and calls being routed to said trunks is less than the number of trunks.

3. In a switching system, trunks extending between ofces and over which interofiice connections may normally be routed, alternate facilities over which connections between offices may be routed, switches for establishing interoiice connections from a calling subscriber at one oiice to a second office at which a called subscribers line terminates, a decoder for controlling the operation of said switches to complete a connection from said rst cnice to said second oilice in response to a, code, said decoder including means to determine the group of trunks over which such connections would normally be routed, means to indicate the busy or idle condition of each trunk of the normal group, means associated with said group of trunks to indicate the number of uncompleted calls in progress toward trunks of said group, a testing circuit for testing the number of trunks of said group over which established calls have been routed, a testing circuit for testing the number of calls already initiated and in progress of being routed to said group of trunks, and means jointly controlled by said testing circuits to cause said decoder to route a call over a trunk of said group when the number of completed calls already routed plus the calls in progress of being routed over trunks of said group is less than the number of said trunks.

4. In a switching system, trunks extending between oflices and over which interofce connections may normally be routed, alternate facilities over which connections between offlces may be routed, switches for establishing interoflice connections from a calling subscriber at one ofce to a second office at which a called subscribers line terminates, a decoder for controlling the operation of said switches to complete a connection from said rst oiiice to said second ollice in response to a, code, said decoder including means to determine the group of trunks over which such connections would normally be routed, means to indicate the busy or idle condition of each trunk of the normal group, means associated with said group of trunks to indicate the number of uncompleted calls in progress toward trunks of said group, a testing circuit for testing the number of trunks of said group over lwhich established calls have been routed, a testing circuit for testing the number of calls already initiated and in progress of being routed to said group of trunks, and means jointly controlled by said testing circuits to cause said decoder to route a call over alternate facilities when the number of completed calls already routed plus the calls in progress of being routed over trunks of said group will utilize all of said trunks.

5. In a switching system, trunks extending loetween offices and over which interoiice connections may lnormally be routed, alternate facilities over which connections between oflices may be routed, switches for establishing interoflice connections from a calling subscriber at one ofce to a second oice at which a called subscribers line terminates, a decoder for controlling the operation oi said switches to complete a connection from said first oiiice to said second cnice in response to a code, said decoder including means to determine the group of trunks over which such connections would normallybe routeda testing circuit. for testing the number of trunks of said group over which established calls have been routed, storage devices for maintaining a record of each call in progress of being routed to said group of trunks until such call is completed to a trunk of the group, a testing circuit for testing the number of records of calls in progress to said group of trunks, and means jointly controlled by said testing circuits to cause said decoder to route a call over a trunk of said group when the number of completed calls already routed plus the calls in progress or being routed over trunks of said group is less than the number of said trunks.

6. In a switching system, trunks extending between offices and over which interoice con- Y* nections may normally be routed, alternate facilities over which connections between olces may be routed, switches for establishing interoice connections from a calling subscriber at one office to a second office at lwhich a called subscri-bers line terminates, a decoder for controlling the: operation of said switches to complete a connection v'from said first-cnice to said second ofce in response to a code, said decoder including means to determine the group of trunks over which such connections would normally be routed, a testing circuit for testing the number oi trunks of said group over which lestalolislfied calls have been routed, storage devices for maintaining a record of each call in progress of being routed. to said. group of trunks until 'such call is completed to a trunk of the group', a testing circuit for testing the .number ofy records of -calls in progress to said group 'of trunks, :and means jointly controlled by said. testing circuits to lcause said-'decoder' 'to routea call over alternate facilities when the Vnumber of completed calls yalready routed lplus thepalls in progress of being routed over `trunks of said group will utilize all of said 7; In a switching system, trunks extending betw'een oiiices and over which interoi'ce connections may normally be routed,y alternate facilities over 'which connections between offices may be routed, y'switches for establishing interoce connections from a calling subscriber at one loil'ice to a second oice lat which a'called subscribers line terminates, a decoder for controlling the operation of saidv switches to complete a connection from. said first cnice tosaid second oiice in response to a code, said decoder including means toV determine the group of trunks over which such connections would normally be routed, a testing circuit for testing the number of trunks of said group over which established calls have been routed, a plurality of condensers, means for maintaining a condition of charge within certain limits upon any of said condensers during such 'time as an 'incompleted call is in progress of f being routed over a trunk of said group, a trigger tube for each condenser. whose operation depends upon the condition of charge of the condenser, devices controlled by each tube responding to theA Voperation thereof, and means con.- 'trolled by the number of. trunks of the set overI which. calls have been routed and by the number of'. tubes operated. to determine whether a call shall. be routed over a trunk of said group or over alternate facilities. .Y

e 8; In a switching system, trunks extending bef t u tween offices and over which int'eroflice connections may normallybe routed, alternate facilities over which connections between oflces may Vbe routed, switches for establishing interoffice connections from a calling subscriber at one ofgedane? ilce to a second ol'ce'at which a called subscribers'line terminateaa decoder forxcontrolling the operation of said switches tov complete'a'connection from said firstv office toV said secondoflice in response to a code, said decoder includ-ing means to `determine the group of trunks over which such connections would normally be routed, atesting circuit for testing the number of trunks of said group over which established calls have been routed, a plurality of condensers, means for maintaining a condition of chargeV within certain limits upon any oi said condensers during such time as an incompleted call is in progress of being routed over a trunk of said group, a trigger tube Ifor eac-h. condenser whose operation de.- pends upon the condition of charge of the condenser, devices controlled by yeach tube respond ing to the voperation thereof, and means controlled by said testing circuit and said vacuum tubes to cause said decoder to route a call over a trunk Qfsaid group when the number of completed calls already routed .plus the calls in. progress of being routed over trunks oi' said group is less than the number of saidV trunks.

9. In a switching system, trunks extending between oices and over which interofce connections may normally be routed, alternate facilities over which connections between oflices may be routed, switches for establishing interoce connections from a calling subscriber at one oce to a second'oiiice at which a called subscribers line terminates, a decoder for controlling the operation of said switches to complete a connection from said firstV on'ice to said second o'ce in response to a code, said decoder including means to determine the group of trunks over which .such connections would normally be routed, a testing circuit for testing the number of trunks of said group over which established calls have been routed, a plurality of condensers, means for maintaining a condition of charge Within certain limits upon any of said condensers during such time as an incompleted call is in progress of being routed over a trunk of said group, a trigger tube for each condenser whose operation depends upon the condition of charge of the condenser, devices controlled by each tube responding to the operation thereof, and means controlled by said testing circuit and said vacuum tubes to cause said decoder to route a call over alternate facilities when the number of completed calls already routed plus the calls in progress of being routed over trunks of said group will utilize all of said trunks. HAROLD I-I.I SCII1=\.OEl?P.EL-.V 

